This invention relates to a device for transverse cutting and welding of webs of thermoplastic plastic, more particularly webs in the form of a tubular film used for the manufacture of bags. The device comprises a pair of feed rolls for feeding the web in steps equal to the length to which the web is to be cut, a transverse cutting knife, preferably parallel to a transverse welding device and preferably heated, and two vertically movable rails placed parallel to each other in the direction of web feed. The rail which is downstream in the direction of feed acts as a gripping rail for pressing the end of the web, fed between the jaws of the transverse cutting device, against a support. The second rail, which is arranged between the cutting knife and the gripping rail, acts as a holding rail for tensioning the web which is gripped between the pair of feed rolls and the gripping rail before the cutting knife cuts the web.
In a device of the above kind, as described in German patent publication 3,204,492 A, it is possible for a section of the web which has been advanced between the jaws of the transverse welding device and under the cutting knife, and which has been held between the pair of feed rolls and the gripping rail, to be so slack that it may not be clamped in the desired manner by lowering the gripping rail.
If insufficient tension is produced by lowering the gripping rail, it is not possible for a severing cut to be produced. Therefore jamming between the cutting edges--as may be caused the heated severing knives--may not be reliably overcome.